This invention relates to a control apparatus for controlling the speed of the car of an elevator, and more particularly to a control apparatus which can limit the car speed to a safe operating level at all times.
In order to operate an elevator car with a good riding quality and with a high floor arrival accuracy, the rotation of an electric motor must be controlled precisely and smoothly. To achieve this objective recent technological progress in microelectronics and power electronics has been applied to elevator systems.
FIG. 5 is a system arrangement diagram showing a control apparatus for an elevator of this type. Referring to the figure, numeral 5 designates a car, numeral 6 a rope, numeral 7 a sheave, numeral 8 a counterweight, and numeral 9 a three-phase induction motor. A pulse generator 10 produces pulses corresponding to the revolution speed of the motor 9, and a counter circuit 11 counts the number of output pulses of the pulse generator 10. A microcomputer 12 is constructed of an input port 121 which forms an interface for receiving a signal from the counter circuit 11, a central processing unit (hereinbelow, termed "CPU") 122, a ROM 123, a RAM 124, an output port 125 which forms an interface for delivering a signal 131 to a power converter circuit 13, and a bus 126. Shown at numeral 14 is a three-phase A.C. power source.
Besides, FIG. 6 is a block diagram showing the function of a feedback control based on the microcomputer 12. A compensator 1 performs phase and gain compensations on the basis of the input of the error .epsilon. between a speed reference signal V.sub.P and a car speed signal V.sub.T, and delivers an output V.sub.C. It has a transfer function G.sub.C (S) where S denotes the Laplace operator. Numeral 4 indicates a converter by which the angular rotational frequency .omega..sub.r of the three-phase induction motor 9, obtained on the basis of the output of the counter circuit 11 received via the input port 121, is converted into the car speed signal V.sub.T (V.sub.T =K.sub.T .multidot..omega..sub.r where K.sub.T denotes a coefficient), and the car speed signal V.sub.T is delivered as an output. Numeral 130 indicates calculation means for converting the output V.sub.C of the compensator 1 into the command value 131 for the power converter circuit 13.
In the control apparatus having the above construction, the pulses corresponding to the rotational frequency of the three-phase induction motor 9 are generated by the pulse generator 10 and are counted by the counter circuit 11, and the count value is transferred to the microcomputer 12. Then, the microcomputer 12 converts the count value into a car speed so as to calculate the car speed signal V.sub.T. Subsequently, it performs the feedback control on the basis of the error .epsilon. between the predetermined speed reference signal V.sub.P and the car speed signal V.sub.T and delivers the command value 131 to the power converter circuit 13. Electric power controlled with this command value is applied to the three-phase induction motor 9, and the speed of the car 5 of the elevator is controlled. That is, the construction of FIGS. 5 and 6 carries out the feedback control by the use of the speed reference signal V.sub.P and the car speed signal V.sub.T, thereby intending to control the speed of the car precisely and smoothly.
With the above construction, however, in a case where the car speed signal V.sub.T presents a value lower than an actual car speed V.sub.car on account of the trouble of the pulse generator 10, the counter circuit 11, the input port 121 or the like, the error .epsilon.(=V.sub.P -V.sub.T) becomes a large value, which, in turn, produces a substantial change in the speed of the car 5, and the car 5 runs recklessly to expose passengers in the car to danger. Such situations are illustrated in FIGS. 7(a) and 7(b). FIG. 7(a) corresponds to the case of a fault which occurs when the car speed signal V.sub.T indicates a zero i.e., when the car 5 stops, while the actual car speed V.sub.car steadily rises. On the other hand, FIG. 7(b) corresponds to the case of a fault which occurs when the car speed signal V.sub.T is clipped to V.sub.S (i.e., after the start of the running of the car 5) while the actual car speed V.sub.car steadily rises. In both cases, the car speed signal V.sub.T indicates a value lower than the actual car speed V.sub.car, and the car speed V.sub.car continues to be increased, that is, the car 5 continues to be accelerated. The difference (V.sub.P -V.sub.T) is increased, causing an error in the command value B1 delivered to the power converter circuit 13 to operate the induction motor 9. As a result, the elevator car runs irresponsively. Finally, a governor (not shown) which is a safety device for preventing an overspeed is operated to stop the car 5. This sudden halt, with the passengers confined in the car, is very dangerous. This drawback is attributed to the fact that the prior-art construction performs the feedback control with the error .epsilon. between the speed reference signal V.sub.P and the car speed signal V.sub.T, thereby to control the torque of the three-phase induction motor 9. For the purpose of avoiding this drawback, it is considered, by way of example, to utilize a double-checked generation means including a pair of pulse generators and a pair of counter circuits for double-checking the rationality of the car speed signal. This measure, however, results in a very expensive and complicated system.